If you stick to the brightly lit parts of the language, things will work. The Facebook codebase was and is enormous, so the vast majority of PHP has to work as advertised. The one big thing that's ruled out is eval, but other wacky PHP tricks ($f(), $C::staticMethod, $obj->$methodName(), foreach order, etc.) work. Outside of eval, you have to go out of your way to break it.
That said, it's a young project, and things can be a bit rocky. Including compilation in your deployment process is also a pain; do not kid yourself about that. It's kind of "industrial strength" in general; unless you care about how many PHP requests you can squeeze out of a unit of hardware, HipHop doesn't have much to offer.
That said, it's a young project, and things can be a bit rocky. Including compilation in your deployment process is also a pain; do not kid yourself about that. It's kind of "industrial strength" in general; unless you care about how many PHP requests you can squeeze out of a unit of hardware, HipHop doesn't have much to offer.